Mixing of a stabilizer and/or an additive into a synthetic resin for improvement of properties of the resin has heretofore been conducted by uniformly mixing a synthetic resin powder with a stabilizer and/or an additive in a Henschel mixer and then heat-melting the resulting mixture in a single or double screw extruder.
In the case of an oxymethylene copolymer, the copolymer immediately after polymerization has thermally unstable terminals --(OCH.sub.2).sub.n --OH! and therefore is generally subjected to terminal stabilization in a molten state by the use of a vent type screw extruder. With respect to the mixing of a stabilizer and/or an additive into a crude oxymethylene copolymer, JP-A-63-196615 discloses completely melting a crude copolymer in an extruder, then mixing therewith a terminal stabilizer and an auxiliary in a pressurized region in the extruder and continuously removing unstable components from the crude copolymer. This publication also discloses in the examples a method for removing unstable components wherein a melt-kneading mixture composed of a stabilizer and/or an additive and polyoxymethylene is introduced into the pressurized region in the extruder.
The position at which and the amount in which the melt-kneading mixture is introduced into the extruder, being disclosed in the above publication, however, results in problems of reduced thermal stability, coloring, etc. of the resulting oxymethylene copolymer, when there is used such a stabilizer and/or an additive as to have reactivity with the terminal stabilizer used or with formaldehyde gas generated during terminal stabilization. Further, since terminal stabilization is often conducted in the vacuum zone of extruder for volatile components removal to intensify the vacuum removal of volatile components for higher terminal stabilization, the stabilizer and/or the additive diffuses into the gas to be vented; thus, the above mixing is also economically disadvantageous.
It is also known to subject a crude oxymethylene copolymer to terminal stabilization and then to feed a stabilizer and/or an additive to a section of an extruder following the vacuum zone for volatile components removal. In this case, however, direct feeding of the stabilizer and/or the additive per se to the copolymer tends to invite poor dispersion of the stabilizer and/or the additive. In order to improve the dispersibility, a means such as making L/D of the extruder larger wherein L is the length of the extruder in the direction of the rotation axis and D is the inner diameter of the extruder, is necessary. This approach is economically disadvantageous as well and tends to invite the thermal deterioration of resin.
Hence, there has been strongly desired a process for producing a stable oxymethylene copolymer, excellent in quality and economy, which is free from the above-mentioned problems.